1. Field of the Invention
The present invention relates to an electronic device having thin film active elements such as thin film transistors, thin film diodes etc.
2. Description of the Related Art
With the recent trend of electronic devices toward light weight, thinner and flexible configurations, technologies of transferring a thin film transistor circuit etc. formed on glass to a resin substrate have been under development. This is done because it is markedly difficult to create a high-performance thin film transistor circuit directly on a resin substrate. Further, in general, the thermal conductivity of glass substrates and resin substrates is very low, so that, concerning the thin film transistor devices formed on these substrates, the problem has come to light in which the characteristics of the device become degraded by the temperature rise of the device which becomes during operation. Some reports on this problem have been found in academic journals (Satoshi Inoue, Satoshi Takenaka and Tatsuya Shimoda, “Study of degradation phenomenon due to a combination of contamination and self-heating in poly-Si thin film transistors fabricated by a low-temperature process”, Japanese Journal of Applied Physics Vol. 42 (2003) pp. 4213-4217).
As to the technologies to address this problem, for example Japanese Patent Application Laid-open 2004-219551 (to be referred to hereinbelow as patent document 1) has proposed a flexible electronic device which is formed by thinning a glass substrate formed with a thin film transistor circuit to be greater than 0 μm to 200 μm in thickness, then by applying a transparent resin film or metal film over its entire surface. In particular, the reason for applying a metal film is to efficiently radiate heat from the self-heating of the device such as thin film transistors etc., to thereby prevent degradation of the device characteristics due to increased temperatures.
Japanese Patent Application Laid-open H11-177102 (to be referred to hereinbelow as patent document 2) has proposed as a structure a thin film transistor device on a glass substrate, in which the size of a silicon active layer is made large so that it functions as a heat sinker to thereby discharge heat in an efficient manner.
Alternatively, Japanese Patent Application Laid-open 2000-137211 (to be referred to hereinbelow as patent document 3) has also disclosed as a technique a thin film transistor liquid crystal display device on a glass substrate, in which large-sized metal heat-radiating pattern elements are connected to the thin film transistor circuits disposed around the substrate so as to enhance the effect of heat radiation.
Further, Japanese Patent Application Laid-open 2001-298169 (to be referred to hereinbelow as patent document 4) discloses a technique for an improved heat radiation effect by forming a thin film transistor device on a sapphire substrate having a higher thermal conductivity than glass.
In the aforementioned prior art, patent document 2 and patent document 3 achieve efficient heat radiation by manipulating the structure of the thin film transistor device on the glass substrate. However, these techniques entail at least two problems. The first is that the size of the thin film transistor device becomes large, hence making it difficult to achieve high integration. Secondly, since the heat sink pattern elements have to be formed outside the thin film transistor array region, extra areas and processes are required.
On the other hand, in patent document 4, heat radiation characteristics of thin film transistors are improved by thinning the sapphire substrate having a relatively high thermal conductivity. However, this method entails the problem that the sapphire substrate is very expensive. Further, though it has a higher thermal conductivity than glass and resin substrates, the heat radiation performance is not sufficient, so that it, after all, this method cannot avoid the temperature rise problem as integration of thin film transistors becomes higher in the future.
Moreover, in patent document 1, heat radiation performance is improved by thinning the glass with thin film transistors formed thereon by etching and applying a high thermal conductivity film of metal etc., on the underside of the glass. However, since the high thermal conductivity film is applied over the entire undersurface, its usage is limited to narrow applications. In particular, it cannot be applied to liquid crystal displays etc., which make use of light transmission.